Preparation of thin film gold based catalysts for oxidation reactions in liquid and gas phases

Thin Solid Films 527 (2013) 96–101

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Preparation of thin film gold based catalysts for oxidation reactions in liquid and gas phases Anne-Félicie Lamic-Humblot a,⁎, Philippe Barthe b, Guillaume Guzman c, 1, Laurent Delannoy a, Catherine Louis a a b c

Laboratoire de Réactivité de Surface, UMR CNRS 7197, UPMC Univ Paris 06, 4 Place Jussieu 75005 Paris, France Philippe Barthe Consultant, 21bis Grande Rue, 77130 Ville Saint Jacques, France Corning CETC, 7 Bis Avenue Valvins 77210 Avon, France

a r t i c l e

i n f o

Article history: Received 31 July 2012 Received in revised form 23 November 2012 Accepted 26 November 2012 Available online 6 December 2012 Keywords: Thin film catalysts Titania Gold Nanoparticles Planar catalysts Carbon monoxide oxidation Benzyl alcohol oxidation

a b s t r a c t This work deals with the preparation of gold on titania catalysts to make catalytic films in the less than 100 nm thickness area and its comparison with usual powder catalyst in catalytic oxidation reactions in gas and liquid phases. Titania was coated on glass plates with different thicknesses, but with ultra-low surface roughness (b5 Å). Gold deposition was performed with usual chemical method for catalysts preparation, that is deposition–precipitation with urea. Transmission electron microscopy showed that planar samples are decorated with a high quantity (>10 wt.% with respect to TiO2) of gold nanoparticles smaller than 2.5 nm, with a narrow size distribution. Activity in CO oxidation demonstrates the catalytic behavior of the planar samples, although they are less active than powder catalyst because of the different geometries of the reactors and catalysts. In contrast, their catalytic performances in liquid phase, benzyl alcohol oxidation, are comparable. These results validate the concept that gold planar catalysts prepared by chemical methods can present similar catalytic behavior as real powder gold catalysts. Such planar catalysts could be useful for bridging the material gap between real and model catalysts in advanced techniques, such as scanning tunnelling microscopy and spectroscopy or high-pressure photoelectron spectroscopy. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Since the prediction of the activity of gold by Hutchings [1] and the discovery of the catalytic activity of gold nanoparticles by Haruta et al. in the 1980s [2], catalysis by gold has grown significantly, especially for oxidation reactions. Studies on model gold based catalysts usually involve Au single crystals, gold-based inversed catalysts, gold-based bimetallic, and Au clusters supported on various substrates. In the last case, samples are in general prepared via the deposition of gold nanoparticles using thermal evaporation of metallic gold on the substrate (oxide single crystals or thin oxide films) and subsequently annealed [3,4]. Such paths of preparation are far from the conditions used for the synthesis of “real” supported gold catalysts which usually involved the deposition of gold precursors in aqueous solution followed by a gas phase reduction. The preparation of planar model catalysts using chemical methods is a challenging task which could help to bridge the material gap between “real” catalysts and metal single crystals by allowing the studies of such planar model catalysts by advanced surface science ⁎ Corresponding author. E-mail address: [email protected] (A.-F. Lamic-Humblot). 1 On leave at Manufacture Michelin, Ladoux Technology Center, 23 Place des Carmes Déchaut, 63040 Clermont-Ferrand, France. 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.11.096

techniques such as in situ scanning tunnelling microscopy and spectroscopy [5–8] or high pressure photoelectron spectroscopy [9–11]. Another advantage of such ultra thin film catalysts would consist in the reduction of heat and mass transport limitations at the surface and inside solid catalysts. For instance, thin-wall reactors encountered a growing interest for heterogeneous gas-phase reactions as they provide good thermal contact with the catalyst in the interior wall. The catalyst can be deposited on the interior wall by a variety of techniques including thin-film deposition by liquid preparation techniques [12–14]. Moreover, oxide ultrathin film supported metal particles may present peculiar catalytic behaviors by comparison with bulk oxide supported systems when the oxide films are deposited on a metallic substrate [15]. The aim of this study was to develop a preparation method allowing to achieve the formation of small gold nanoparticles deposited on a planar substrate via the classical deposition–precipitation (DP) process used for the synthesis of supported gold catalyst on powder oxide supports. The substrate consists of a glass surface coated by a film of oxide (TiO2) of different textures and thicknesses. The challenge was especially to obtain a high density of small gold particles in spite of the small geometric area of the substrate. In order to validate the preparation method, the catalytic activity of such planar catalysts was investigated in two reactions, one in gas phase, CO oxidation at atmospheric pressure, and one in the liquid phase, the

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oxidation of benzyl alcohol under high pressure. Both reactions have been widely studied for gold catalysts [16,17].

2. Experimental details 2.1. TiO2 coating on glass plates TiO2 films were coated on glass plates (Corning EagleXG, 0.07 cm× 2 cm× 5 cm) according to dip coating procedure developed by Corning. It is based on hydrolysis via an esterification reaction: R-OH+ R′-COOH↔H2O+RCOOR′ and complexation via chelating ligand such as 2,4-pentanedione [CH3COCH2COCH3]. The glass plates have a surface roughness of 0.45 nm, and contain no alkaline element. Before coating, the plates were washed carefully with laboratory soap and abundantly rinsed with distilled water and dried under infrared heating. Dip coating was performed according to the following general procedure for titania: titanium tetra-n-propoxide was added to isopropanol in a glass bottle and mixed in a stirring plate for 15 min resulting in a homogeneous solution. Still under stirring, pentanedione was added and mixed for 1 h. Finally, acetic acid was added and mixed for 30 min. The bottle was closed with a cap and introduced in an oven at 60 °C overnight (about 17 h). Then the thin films of precursor solutions were deposited by dip-coating technique under controlled withdrawal speed. Finally, the films were dried at 60 °C for 1 h then calcined in air in a furnace at 550 °C for 1 h after a ramp up at 300 °C/h. Three different TiO2 coatings were prepared under the different conditions gathered in Table 1: one so-called “dense” that corresponds to a 70 nm-thick coating, one so-called “nanotextured” (NanoT) that corresponds to a 100 nm-thick coating, and one so-called “ultrathin” (UThin) that corresponds to a 7 nm-thick coating.

2.2. Gold deposition on TiO2-coated glass The deposition of gold on the coated plates was performed according to an adaptation of the classical method of DP with urea used for powder oxide support [18,19]. The method is based on the controlled precipitation of gold–urea complex onto the support from HAuCl4.3H2O in aqueous solution, owing to the gradual increase of pH resulting from the thermal decomposition of urea [19,20]. A desired volume (80 μL) of an aqueous solution of HAuCl4.3H2O (Acros, 2.5 × 10 −2 M) was added to 100 mL of distilled water heated at 80 °C. The TiO2 coated glass plate was immerged in this solution and a solution containing 0.5 g of urea dissolved in 10 mL of distilled water was added dropwise using a syringe pump (0.5 mL/min) under magnetic stirring in order to increase the pH very slowly and to favor the deposition of gold onto the support [21]. After 1 h under stirring, the sample was thoroughly washed with distilled water, dried under vacuum at RT (room temperature) for 12 h, and stored in the dark in a dessicator, resulting in the so-called as-prepared samples. The reduction of gold (III) species into metallic gold nanoparticles for the as-prepared Au/TiO2-glass samples was performed by calcination in a muffle furnace from RT to 300 °C at 2°/min, then 4 h at 300 °C. Table 1 Conditions of preparation of the titania thin films on glass.

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2.3. Preparation of reference catalysts on powder oxides As a reference Au/TiO2 powder catalysts with 1 wt.% Au were prepared according to the classical method of DP with urea [18]. 1 g of TiO2 (Degussa, 50 m2/g) was added to 100 mL of an aqueous solution of HAuCl4 (4.2×10−3 M) and urea (0.42 M). The suspension thermostated at 80 °C was vigorously stirred for 4 h. Then, the solid was centrifuged, washed three times with water and dried at RT. Activation of the powder catalysts was performed in a tubular fixed-bed reactor under a flow of air (100 mL/min−1) from RT to 300 °C at 2°/min, then 2 h at 300 °C. 2.4. Characterization techniques The films were characterized by scanning electron microscopy (SEM) (Leo 1550 à 3–5 kV). Before analysis, the samples were metalized with 2 nm Iridium. The X-ray diffraction (XRD) patterns were collected by using a Philips X-Pert Pro diffractometer equipped with a copper tube mounted in Kα1 line (collimator before germanium) at the power of 45 kV/40 mA (λ = 1.540593). The angle was varied from 24 to 60°, with a step of 0.008°, and a time per step of 500 s. The ellipsometry apparatus used was a SENTECH SE800 (spectroscopic, 380–780 nm). The surface of the TiO2 thin films was characterized by wide light interference (WLI) (Zygo instrument) and atomic force microscopy (AFM) (VEECO Autoprobe M5). The chemical titration of gold and titanium in the Au/TiO2-glass was performed as follows: after selective extraction of the coating with aqua regia, the solution analysis was made by ICP MS (inductively coupled plasma-mass spectrometry) in Corning Research Center to determine the gold and titanium loadings as well as the total mass of titania and gold per plate. The chemical titration of gold on titania in the powder samples was performed by ICP AES (inductively coupled plasma atom emission spectroscopy) at the CNRS Centre of Chemical Analysis (Vernaison, France). For transmission electron microscopy (TEM) analyses, sampling of the Au/TiO2 coating of the planar samples was performed as follows: a drop of ethanol was put onto the plate then the surface of the plate was scraped with a razor blade. The drop of ethanol containing the sample was deposited on a carbon-coated copper grid. TEM images were collected with a JEOL 2010 microscope operating at 200 kV equipped with a charge-coupled device camera. Particle size measurements were performed particle by particle, using iTEM Soft Imaging System software (Olympus Soft Imaging Solutions). The limit of size detection was about 1 nm. The average particle size values were obtained by measuring the diameters of about 800 particles on several images, coming from different regions of the sample grid and by using the following formula: dAu = Σnidi/Σni (arithmetic mean) where ni is the number of particles of diameter di. X-ray photoelectron spectra (XPS) were collected on a SPECS (Phoibos MCD 150) spectrometer, using a Mg Kα X-ray source (hν = 1253.6 eV) having a 150 W (12 mA, 12.5 kV) electron beam power and a 7 × 20 mm spot size. The emission of photoelectrons from the sample was analyzed at a takeoff angle of 90° under ultra high vacuum conditions (1 × 10 −8 Pa). XPS spectra were collected at pass energy of 15 eV for C1s, Pt4f and Au4f core XPS levels. No charge compensation was applied during acquisition. After collection, the binding energies were calibrated with respect to the C 1 s peak at a binding energy of 284.8 eV [22]. Spectra processing were carried out using the Casa XPS software package. The atomic ratios were calculated after normalization using Scofield factors [23].

Oxide film on glass

Mass of titanium n-propoxyde (g)

Volume of isopropanol (mL)

Mass of pentanedione (g)

Mass of acetic acid (g)

Dip-coating rate (cm/min)

2.5. Catalytic reactions

TiO2-dense TiO2-NanoT TiO2-UThin

10.26 10.26 1.137

48.25 48.25 78

7.01 7.01 0.815

4.2 0 0.484

6 3 3

Oxidation of CO over the planar catalysts was performed in a glass reactor specifically designed to vertically enclose the plates with the gas flow passing over the plates. As in the case of the powder catalysts, the planar catalysts (one plate in the reactor) were activated

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Table 2 Physical characteristics of the oxide films. Oxide film on glass

Film thickness (nm)

crystallite sizea (nm)

Crystallographic structure

TiO2-dense TiO2-NanoT TiO2-UThin

70 100 7b

15 15 10–15

Anatase Anatase –

a b

Determined by SEM and/or XRD. Determined by ellipsometry.

at 2°/min. The reaction was performed at 130 °C for about 24 h. 1 mL of liquid was withdrawn through a sample loop for gas chromatography analysis (Agilent technologies, GC7890 with flame ionization detector, column HP5M5 30 m × 250 μm × 0.25 μm). The first aliquot was taken at t = 0, i.e., just after the reactor was pressurized, then every hour. Only benzaldehyde and benzoic acid were observed.

3. Results and discussion 3.1. Characterization of the oxide films

in situ in 20 vol.% O2/He mixture (100 mL/min) at 300 °C (2 °C/min) for 3 h. After cooling down to RT under the pre-treated gas, a mixture of 1 vol.% CO and 10 vol.% O2 in He (50 cm 3/min) was introduced in the reactor. The temperature was linearly increased from room temperature to 300 °C at 2 °C/min. The reactor outflow was analyzed using an infrared detector (Maihak 710), allowing the simultaneous quantification of CO and CO2. Catalytic oxidation of benzyl alcohol was performed in 300 mL stainless steel autoclave reactor (Parr Instruments) equipped with a mechanical bar for stirring. The catalysts (5 glass plates or 0.1 g of powder catalysts) calcined ex situ under air were placed in the autoclave, and were activated under the air from RT to 200 °C, 2 °C/min ramp and 2 h at 200 °C. After cooling at RT, 4 mL of benzyl alcohol (0.038 mol) diluted in 131 mL of distilled water were introduced into the reactor. The reactor was pressurized with air at 2 × 10 6 Pa (0.47 mol of O2). The temperature was increased from RT to 130 °C

The TiO2-dense film presents an anatase crystalline structure as determined by X-ray diffraction; from the [101] diffraction at 2θ = 25.4°, a crystallite size of 15 nm was determined for TiO2-dense film. The same structure was observed for TiO2-NanoT with a crystallite size of 13 nm. No diffraction could be observed for TiO2-UThin, probably because the amount of titania on the plate was too low (Table 2) or because the film becomes amorphous with the thickness [24]. SEM was used in order to determine the thickness of the film and the surface state of the samples. Fig. 1(a),(b),(d),(f) shows the SEM images of TiO2-dense and TiO2-NanoT. The surface of the TiO2-dense films is very uniform, showing smooth grained structure. On the opposite, TiO2-NanoT exhibits a bumpy surface, with craters. The grain size is similar to the size determined with XRD indicating that each TiO2 grain is composed of one crystallite.

Fig. 1. (a) Surface texture of TiO2-dense film, (b) SEM cross-section of TiO2-films deposited with a dipping speed of 6 cm/min, (c) AFM image of TiO2-dense film, (d) SEM surface view of the AFM scanned area, (e) SEM surface view of TiO2-NanoT film, (f) detailed SEM surface view of TiO2-NanoT film, and (g) SEM surface view of TiO2-UThin film at high magnification.

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Scheme 1. Scheme of the grain texture of TiO2 ultra-thin films.

The thickness of the film was also evaluated by SEM and was found to be 70 nm for TiO2-dense film and around 100 nm for TiO2-NanoT film. The data are summarized in Table 1. The thickness of TiO2-UThin was determined by ellipsometry. After several Cauchy simulations of the ellipsometry data performed to determine the best fit of film thickness and refractive index, a minimum film thickness of 4.27 nm and refractive index of 2.03 were obtained. This thickness is smaller than the grain size measured from surface SEM view (7 nm). A possible explanation is that the TiO2 grains are flattened on the glass plate. A schematic illustration of the TiO2-UThin film nanostructure is given in Scheme 1. The surface roughness of the TiO2-dense film was characterized by AFM technique (Fig. 1(c)). The image shows that TiO2-dense film exhibits extremely low roughness (b5 Å).

3.2. Characterization of the planar catalysts after deposition of gold The amount of gold deposited on planar substrate depends on the nature of the oxide film (Table 3). It is similar for the thick films, TiO2-dense and TiO2-NanoT, which leads to the same gold loadings (Au/TiO2 weight ratio) of 10–11 wt.%. In contrast, the amount of gold is much smaller on TiO2-UThin, but as the amount of titania is also very low, a much higher gold loading (35 wt.%) is achieved. As expected from previous studies [25], the gold loading on the powder support is 1 wt.%. It was not possible to determine the distribution of gold particles on the oxide films by SEM because the size of the gold particles was too small. A sampling was thus performed by scraping the surface of the plate to perform TEM measurements. The average gold particle sizes deduced from the TEM measurements are reported in Table 3. In all cases, the average gold particle is around 2 nm. Fig. 2 shows a typical TEM image of Au/TiO2-glass-dense; the gold particles are small and homogeneous in size, and very well dispersed on the titania support in spite of the high density of gold particles. This was also observed in the other TiO2 samples. It is worth to underline that the size of gold particles obtained using the DP method is equivalent to the smallest size achieved using other chemical methods to prepare planar model catalysts. Indeed, gold nanoparticles (2–5.5 nm) prepared via colloidal routes were deposited on SiO2/Si or TiO2/Si wafers using the Langmuir–Blodgett (LB) method [11] for in situ XPS study. Inverse micelle encapsulation was also used to prepare gold nanoparticles

Fig. 2. TEM image of Au/TiO2-dense (scratched from glass) and corresponding particle size distribution.

(1.5–6 nm) for scanning tunnelling microscopy and spectroscopy investigations [6]. The samples pre-treated at 300 °C under air (Au/TiO2) were analyzed by XPS [25]. For all samples, only gold in the metallic state could be identified with a doublet at 84.1 eV (Au 4f7/2) and 87.7 eV (Au 4f5/2) (Fig. 3); gold in higher oxidation state (Au III) is expected to display some peaks at higher binding energies (85.6 eV for Au 4f7/2 and 89.2 eV for Au 4f5/2) [26,27]. The Au4f/Ti2p ratio varies with the nature of the oxide film (Table 3). Au/TiO2-UThin shows the highest Au/M ratio (1.5), which is consistent

Table 3 Characteristics of the Au/TiO2 samples: Gold loadings, particle size (TEM) and dispersion (XPS). Sample Au/TiO2-dense Au/TiO2-NanoT Au/TiO2-UThin Au/TiO2-powder ⁎ Standard. deviation. a Au/M=Au4f/Ti2p.

Film thickness (nm) 70 100 7 –

Mass of gold (μg) 83 95 17 –

Mass of oxide (μg) 695 745 32 –

Gold loading (wt.%) 10.7 11.3 34.7 0.97

Gold particle size (nm) 2.4 1.8 2.1 2.1

(0.76) ⁎ (0.57) (0.83) (0.80)

XPS Au/M 0.27 0.85 1.48 0.034

Ref a

This work This work This work [32]

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geometry of the two reactors employed is different, the reactant gases flow over the catalyst plates and not through the catalyst bed as in the case of the powders. Therefore, the contact time between the Au/TiO2 films and the reactants is considerably lower than in the case of the powder samples (4 × 10 −5 s vs 3 × 10 −3 s). In addition, gas-phase diffusion limitations have been proposed to interpret the higher temperature required to obtain CO conversion on Pt/ZrO2 planar catalysts as compared to the powder ones [29]. Despite these unfavorable conditions, CO conversion can be achieved at higher temperature. Indeed, although Au/TiO2-UThin remains inactive for up to 300 °C, CO2 formation starts to be detected at 200 °C for Au/TiO2-NanoT and 150 °C for Au/TiO2-dense. The lack of activity of Au/TiO2-UThin might not result from the low amount of gold present in the catalyst since the IR detector is able to detect the formation of very low amount of CO2 (200 ppm). An explanation could be that ultrathin thickness of the titania film (7 nm) would have modified the intrinsic properties of titania; indeed the titania particles are flattened as shown in Scheme 1, moreover the structure of titania could not be identified. The consequence could be that the gold–titania interaction, which is crucial for gold catalysts activity in CO oxidation, is different in the Au/TiO2-UThin ([30] and references there in). Au/TiO2-dense and Au/TiO2-NanoT show comparable activity at 300 °C. As with the film thickness, the gold loading and the gold nanoparticle size are close, it appears that the TiO2 film morphology does not seem to have an influence on the activity of the planar catalysts. 3.4. Benzyl alcohol oxidation

Fig. 3. (a) XPS spectrum of gold on Au/TiO2-NanoT after thermal treatment. The red line represents the experimental XPS spectrum. The black line is the baseline for the deconvolution of the spectra. The blue line corresponds to the Au 4f5/2 peak and the purple one to the Au 4f7/2 peak and (b) XPS C 1 s peak used for internal calibration of the binding energy. The black line is the baseline for the deconvolution of the spectra. The purple line corresponds to the C 1 s peak.

with the higher gold loading determined by elemental analysis, and with the fact that in contrast to the other samples, all gold is detected by XPS since the thickness of the entire film (7 nm) is close to the mean free path of the X-rays, which is around 7–8 nm [28]. The Au4f/Ti2p ratio is higher for Au/TiO2-NanoT (0.85) than in Au/ TiO2-dense (0.27). This might be explained by the fact that during DP [3,18], the solution containing the Au III precursor soaks the whole film, therefore the AuIII precursor can interact with the entire opened surface of titania and the Au 0 particles formed during reduction are evenly distributed within the whole thickness of the film. It is expected that since TiO2-NanoT is more porous than TiO2-dense, the penetration of the X-ray beam is facilitated and more gold can be detected. 3.3. CO oxidation reaction The planar catalysts do not show any activity at room temperature in CO oxidation in contrast to the powder catalysts (Table 4). According to a former study performed under the same conditions [25], the activity of 1 wt.% Au/TiO2 powder at RT is equal to 1.08 molCO/s/molAu after 2 min of reaction, i.e. at the maximum of activity before progressive deactivation. At least two reasons may be invoked to explain the absence of reactivity of the planar catalysts. The amount of gold on a plate is smaller (b 100 μg, Table 3) than in the 50 mg used for the powder catalysts (500 μg). Moreover, the

In the case of the catalytic oxidation of benzyl alcohol, performed in liquid phase and under high pressure, the comparison of the planar and powder catalysts is more straightforward as both kinds of catalysts are tested in the same conditions. As shown in Table 5, the planar catalysts are active in this reaction and even more active than Au/TiO2 powder. Moreover, all the samples present a selectivity to benzaldehyde higher than 90% after 24 h reaction, which is comparable to the selectivity obtained after 3 h reaction in pure benzyl alcohol [31]. The different activity for planar and powder catalysts can be explained by the different geometries of the catalysts. The contact time of the reactant on powder Au/TiO2 is certainly shorter than on planar Au/TiO2 since it is dispersed in the solution and titania is not porous. In the case of the planar samples, the reactant may be “blocked” in the porosity of the film, which results in a longer contact time, therefore in a higher activity and lower selectivity. This may also explain that Au/TiO2-NanoT is more active than Au/TiO2-dense; its thickness is higher and the contact time increases. Table 5 also gives the selectivity in benzaldehyde (the desired product) and benzoic acid. All the samples present selectivity in benzaldehyde higher than 90%. Au/TiO2-dense is more selective than Au/ TiO2-NanoT. Selectivity decreases as activity increases. We can also note that Au/TiO2-dense is more selective than Au/TiO2 powder, maybe because the transfer of matter is better in the planar catalysts. 4. Conclusion This work demonstrates that it is possible to generate a high density of small gold particles on an oxide film using the chemical methods of preparation used usually for the preparation of powder

Table 4 Activity in CO oxidation for the different Au/TiO2 planar catalysts. Catalyst

Gold particle size (nm)

Activity in CO oxidation at RT/at 300 °C (molCO/s/molAu)

Au/TiO2-dense Au/TiO2-NanoT Au/TiO2-UThin

2.4 1.8 2.1

0/2.5 × 10−1 0/9.7 × 10−2 0/0

A.-F. Lamic-Humblot et al. / Thin Solid Films 527 (2013) 96–101 Table 5 Catalytic properties of the different Au/TiO2 catalysts in liquid phase oxidation of benzyl alcohol. Reaction conditions: 4 mL benzyl alcohol in 131 mL water, 0.1 g catalyst (powder) or 2 glass plates, 130 °C, 20 bar air (4 bar O2), 1000 rpm, after 24 h reaction. Catalyst

Au/TiO2-NanoT Au/TiO2-dense Au/TiO2 powder

Activity (molbenz. 10,184 7828 5662

Selectivity (%) alc. consumed/molAu)

Benzaldehyde

Benzoic acid

90.4 98.3 91.7

9.6 1.7 8.3

catalysts. It also shows that such catalysts can be active in gas phase, although the reaction conditions of CO oxidation were not optimized, and in liquid phase reactions. These results validate the concept that planar catalysts can present similar catalytic behavior than the usual powder catalysts. These planar catalysts could be useful in bridging the gap between real and model catalysts and could be studied by advanced techniques, such as scanning tunnelling microscopy and spectroscopy or high pressure photoelectron spectroscopy. Acknowledgment We would like to thank Corning SA for financial support and Jean-Jacques Théron for the complementary informations. References [1] [2] [3] [4]

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